1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a liquid crystal display (LCD) device and a method for manufacturing the same that improves reflexibility by maximizing the density of protrusions.
2. Description of the Background Art
With the development of an information society, demands for various display devices are increasing. Accordingly, many efforts have been made to research and develop various flat display devices such as liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD) and vacuum fluorescent display (VFD) devices. Some of these types of flat display devices are already applied to displays of various equipments.
Among the various flat display devices, the LCD device has been most widely used due to the advantageous characteristics of being thin, light in weight, and having a relatively low power consumption. The LCD device substitutes for the Cathode Ray Tube (CRT), and in addition to the mobile type LCD devices, e.g., such as a display for a notebook computer, the LCD devices have been developed for computer monitors and televisions to receive and display broadcasting signals.
Despite various technical developments in the LCD technology and associated applications in different fields, research in enhancing the picture quality of the LCD device has been in some respects lacking, e.g., as compared to other features and advantages of the LCD device. Therefore, in order to use the LCD device in various fields as a general display, the key to developing the LCD device lies on whether the LCD device can implement a high quality picture, e.g., such as retaining high resolution and high luminance with a large-sized screen while still being light in weight, thin, and requiring low power consumption.
The LCD device displays an image or a picture by controlling a light transmittance with an electric field applied to the liquid crystal having dielectric anisotropy. The LCD device is different from display devices such as an electroluminescence (EL) device, a cathode ray tube (CRT) and a light emitting diode (LED) device, which emit light by itself, in that the LCD device makes use of ambient light as a light source.
The LCD devices are classified into two different types, a transmitting type LCD device and a reflective type LCD device. The transmitting type LCD device has a backlight as a light source at the rear of an LCD panel, whereby the transmitting type LCD device can display colors by controlling the light transmittance incident on the liquid crystal according to the alignment of the liquid crystal. However, the transmitting type LCD device has problems in that it requires a relatively high power consumption. Meanwhile, the reflective type LCD device makes use of ambient light as the light source, thereby requiring a relatively small amount of power consumption.
FIG. 1 is a cross-sectional view illustrating a general reflective type LCD device of the background art. Referring to FIG. 1, the general reflective type LCD device includes an upper substrate 13 having a color filter layer (not shown) and a common electrode 17, a lower substrate 11 having a thin film transistor (not shown) and a reflective electrode 16, and a liquid crystal 19 between the lower and upper substrates 11 and 13. The liquid crystal 19 is the optical anisotropy medium controlling the light transmittance by aligning liquid crystal molecules in a predetermined direction according to the electric field. Herein, it is possible to use a predetermined medium having the optical anisotropy characteristics instead of the liquid crystal 19.
After that, a plurality of optical medium layers are formed on external surfaces of the respective lower and upper substrates 11 and 13 to control the polarizing state of light. For example, a light-scattering film 21, a phase difference plate 23 and a polarizing plate 25 are sequentially deposited on the upper substrate 13. Herein, the light-scattering film 21 is formed so as to provide a wide viewing angle for a viewer by scattering light, and the phase difference plate 23 includes a first phase difference film having characteristics of λ/4 plate to affect the light to the reflective electrode, and a second phase difference film having characteristics of λ/2 plate. When a voltage is not applied in a turn-off state, the phase of the light is inverted by the phase difference plate 23, thereby obtaining a phase difference. Thus, it is possible to emit a large amount of light to the outside so as to obtain an LCD panel having high luminance characteristics. Also, the polarizing plate 25 transmits the light at a wave direction of a transmitting axis, and absorbs the rest of the light.
Hereinafter, a reflective type LCD device of the background art and a method for manufacturing the same will be described with reference to the accompanying drawings. FIG. 2 is a plan view illustrating a reflective type LCD device of the background art having a reflective electrode including a protrusion, and FIG. 3 is a cross-sectional view taken along line IV—IV of FIG. 2.
As shown in FIG. 2 and FIG. 3, a thin film transistor T is formed on a predetermined portion of a lower substrate 11 by a common technology, and a passivation layer 36 is formed on the lower substrate 11 having the thin film transistor T. Then, a plurality of photo-acryl protrusions 37a are formed on the passivation layer 36 at fixed intervals. At this time, the protrusions 37a are formed on an entire surface of the lower substrate 11 including the thin film transistor T at the fixed intervals, thereby improving a reflection angle of light.
A reflective electrode 16 is then formed for being electrically connected with a drain electrode of the thin film transistor on the passivation layer 36 including the protrusions 37a. At this time, the reflective electrode 16 has an uneven surface by the protrusions 37a formed on the passivation layer 36, so that it is possible to concentrate the light incident on the protrusions 37a at different angles, and to emit the light concentrated at a predetermined angle in case the light incident from the outside is reflected and emitted. An organic insulating layer 38 is formed on an entire surface of the lower substrate 11 including the protrusions 37a, and the reflective electrode 16 is formed on the organic insulating layer 38.
FIG. 4A to FIG. 4E are cross-sectional views illustrating manufacturing process steps of a reflective type LCD device of the background art taken along line IV—IV of FIG. 2. As shown in FIG. 4A, the passivation layer 36 is formed on the entire surface of the lower substrate 11 including the thin film transistor (a gate electrode 27, a gate insulating layer 28, a source electrode 29, a drain electrode 31 and a semiconductor layer 30), and a first photo-acryl layer 37 is formed on the passivation layer 36. Subsequently, a diffraction mask 39 defined by a closed area A, a slit area B and a transmission area C is aligned above the first photo-acryl layer 37.
Referring to FIG. 4B, a diffraction exposure and developing process is performed to the first photo-acryl layer 37, thereby patterning a plurality of first photo-acryl patterns 37b at fixed intervals. By the diffraction mask 39, each of the first photo-acryl patterns 37b has a different thickness in the central and side portions thereof. That is, the portions of the first photo-acryl layer 37 corresponding to the slit area B and the transmission area C of the diffraction mask 39 are exposed differently to the light, whereby the central portion of the photo-acryl layer 37 has a different thickness as that of the side portion thereof.
As shown in FIG. 4C, the hemisphere-shaped protrusions 37a are patterned by reflowing the first photo-acryl patterns 37b in a thermal treatment. Referring to FIG. 4D, a second photo-acryl layer 38 is formed on the entire surface of the lower substrate 11 including the hemisphere-shaped protrusions 37a. Then, the second photo-acryl layer 38 and the passivation layer 36 are selectively removed to expose the predetermined portion corresponding to the drain electrode 31 of the thin film transistor by photolithography, thereby forming a contact hole 35. At this time, the hemisphere-shaped protrusions 37a scatter the light greatly, so that the reflexibility of the light becomes low. In this respect, the second photo-acryl layer 38 is formed at a thickness between 1.5 μm and 2.0 μm, and then a thermo-hardening process is performed thereon, whereby the second photo-acryl layer 38 flows along the surface of the protrusions 37a. Accordingly, the second photo-acryl layer 38 fills up the space between the protrusions 37a, and the height of the hollow space between the protrusions 37a becomes lower, so that it is possible to obtain the protrusion having the height and the radius at a ratio of 1 to 10.
As shown in FIG. 4E, a conductive opaque metal layer such as aluminum Al having great reflexibility is deposited on the entire surface of the lower substrate 11 including the contact hole 35. Subsequently, the conductive opaque metal layer is selectively removed by photolithography, whereby the reflective electrode 16 is formed in a pixel region, for being in contact with the drain electrode 31.
In the reflective type LCD device of the background art, each of the photo-acryl patterns 37b has a different thickness in the central and side portions thereof by the diffraction exposure of the diffraction mask 39. That is, the step difference is generated in each of the photo-acryl patterns 37b by exposing the portions of the photo-acryl pattern differently to the light. Then, a melt-bake process is performed to the photo-acryl patterns 37b, thereby forming the hemisphere-shaped protrusions 37a by thermal-flow.
However, the present inventor has determined that the method for manufacturing the reflective type LCD device of the background art has the following disadvantages. In the method for manufacturing the reflective type LCD device of the background art, the specific exposure method such as the diffraction exposure process is performed as required to obtain accuracy of the diffraction mask.
Also, in the aforementioned method for manufacturing the reflective type LCD device of the background art, it is possible to decrease the hollow space between the protrusions in height by the second photo-acryl layer. However, it has the problem in that the manufacturing process steps are complicated and complex due to the thermal-flow characteristics. Also, during the process for forming the protrusions, it is required to maintain a predetermined space between the protrusions so as to avoid interference therebetween. Accordingly, the density of the protrusions becomes low.
In addition, as shown in FIG. 11A, an inflection point V exists in a curved surface profile of the protrusion by the second photo-acryl layer 38 formed on the entire surface including the protrusions 37a. As the inflection point V is positioned at the center of the protrusion, it has the problem of increasing mirror reflection.